Polyisocyanates have had wide study as adhesives and binders in forming composite materials. For example, polyisocyanates have been used to manufacture composite wood products such as hardboard and particle board. Each such product has a defined scope of industrial utility, rendering it useful for certain applications but not others.
U.S. Pat. No. 3,666,593 to Lee discloses a method for making rigid panels comprising a rigid substrate overlaid with one or more layers of polyisocyanate-impregnated paper. Candidate substrates are wood, metal, plastic, or asbestos board. Lee teaches that at least 30% (up to about 150%) w/w polyisocyanate resin, relative to the mass of the paper, is required to yield a satisfactory paper-overlay panel. Lee also teaches that the polyisocyanate-impregnated paper should be cured at a pressure ranging from 100 to 350 psi and at a temperature within the range 280.degree. to 340.degree. F.
U.S. Pat. No. 4,359,507 to Gaul et al. discloses an adhesive binder composition suitable for lignified cellulose for manufacturing composite materials such as flake board or particle board. The binder comprises a polyisocyanate and an amount of a mixture of ethylene carbonate, propylene carbonate, and optionally extraneous lignin sufficient to lower the viscosity of the binder. The amount of polyisocyanate is within the range of 20 to 95% w/w, preferably 50 to 75% w/w, relative to the mass of the mixture of polyisocyanate and carbonate. The purpose of the carbonates and optional lignin is to reduce the viscosity of the binder. A less viscous binder is apparently easier to apply to lignified cellulose particles such as wood chips and the like during a milling, mixing, or tumbling operation. The carbonates and lignin also serve as extenders to reduce the requisite amount of relatively expensive polyisocyanate needed for satisfactory binding together of the lignocellulose particles.
U.S. Pat. No. 4,486,557 to Gaul et al. discloses an adhesive binder composition for lignified cellulose comprising a mixture of a polyisocyanate, an epoxide, and lignin. Suitable lignified celluloses include wood chips and the like as in the U.S. '507 patent. The lignin and epoxide serve as diluents and extenders for the polyisocyanate.
U.S. Pat. No. 4,344,798 to Gaul et al. is similar to the U.S. '557 patent except that the binder comprises a polyisocyanate and a dialkyl ester of carbonic acid and optionally lignin. The latter two ingredients serve as extenders for the polyisocyanate.
U.S. Pat. No. 4,361,662 to Gaul et al. is similar to the U.S. '557 patent except that the binder comprises a polyisocyanate-alkylene oxide or halogenated alkylene oxide. Again, the additional ingredients serve as polyisocyanate extenders to reduce cost.
U.S Pat. No. 4,414,361 to Gaul et al. is similar to the U.S. '507 patent except that the binder includes, along with a polyisocyanate, a cyclic alkylene carbonate and optionally lignin as extenders to reduce the requisite amount of costly polyisocyanate.
A portion of the teachings of Gaul et al. as disclosed in the U.S. '662 and '361 patents is summarized in Gaul et al., "Novel Isocyanate Binder Systems for Composite Wood Panels," Proceedings of the S.P.I. International Technical/Marketing Conference, San Diego, Calif., Nov. 2-4, 1983, pp. 389-407. In that report, propylene carbonate is described as an "effective diluent" for isocyanates, offering distinct "cost advantages" when used with a polyisocyanate such as poly(diphenylmethane diisocyanate) ("PMDI") in, for example, a 70% PMDI/30% propylene carbonate binder system. That report also summarizes certain physical properties of propylene carbonate, such as a high boiling point of 242.degree. C. (at 760 mmHg) and a high flash point of 132.degree. C. Further, that report teaches that dilution of PMDI with organic liquids miscible in the polyisocyanate, such as propylene carbonate, yields particle boards exhibiting decreased internal bonding strength, where the greater the percent of propylene carbonate relative to the PMDI, the lower the internal bonding strength. PMDI dilution also results in decreased modulus of rupture and modulus of elasticity.
Finally, European patent application No. 0,304,273 to Younes discloses laminate composite materials in which at least one substance such as metal, cellulose, plastic, glass, or ceramic is compression-molded with a "reaction mixture" of a polyisocyanate and a cyclic alkylene carbonate. The amount of cyclic alkylene carbonate is within the range of 2 to 50% w/w, preferably 20 to 35% w/w, relative to the mass of the isocyanate-carbonate mixture. The "reaction mixture" also includes a special catalyst comprising a soluble adduct of a tertiary amine and a cyclic alkylene carbonate. Cellulosic composites producible according to this publication include woodlike materials and particle boards. The polyisocyanate component of the "reaction mixture" apparently forms a rigid polyisocyanurate thermoset rather than a polyurea.
Hence, the prior art teaches various schemes for producing composites from cellulosic materials and polyisocyanate, where a miscible compound in the reaction mixture such as propylene carbonate or other organic liquid serves as an extender or diluent, allowing the use of lesser amounts of costly polyisocyanate. While the prior art suggests that it is desirable to add as much extender as possible from the standpoint of cost, the maximal amount of extender relative to the amount of polyisocyanate is limited, according to the prior art, by the maximum allowable compromise in physical properties of the composite resulting from diluting the polyisocyanate. In other words, the prior art teaches that diluting the polyisocyanate, while lowering manufacturing costs of particle boards and the like, also yields decreased values of important physical parameters such as internal bonding strength, modulus of rupture, and modulus of elasticity. As a result, composites made according to the teachings of the prior art, while having a certain range of useful properties, lack utility for other important industrial uses.
Accordingly, there is a need for industrially useful composites of polyurea and cellulosics other than wood chips and the like, particularly composites comprising substantially delignified fibrous cellulose fillers such as paper-like materials.
There is also a need for such composites wherein the mass percentage of the polyurea is small relative to the mass of the cellulosic material, such as 20% w/w or less.
There is also a need for a process for making such composites wherein the polyisocyanate can be diluted with a miscible organic solvent to reduce manufacturing costs, while still forming a composite having physical properties that are equal to or exceed the corresponding physical properties of similar composites made using more costly undiluted polyisocyanate.
There is also a need for such composites having a substantially uniform distribution of polyurea throughout, even when the polyisocyanate is applied as a liquid resin to the outside surfaces of the substantially delignified cellulose and must subsequently penetrate into the interior matrices of the cellulose material before the polyisocyanate is cured to form the composite.
There is also a need for such composites that can be made having multiple plies each comprised of substantially delignified cellulose and polyurea, where the distribution of polyurea is substantially uniform throughout the composite.